Auto merge of #116846 - krtab:slice_compare_no_memcmp_opt, r=the8472

A more efficient slice comparison implementation for T: !BytewiseEq

(This is a follow up PR on #113654)

This PR changes the implementation for `[T]` slice comparison when `T: !BytewiseEq`. The previous implementation using zip was not optimized properly by the compiler, which didn't leverage the fact that both length were equal. Performance improvements are for example 20% when testing that `[Some(0_u64); 4096].as_slice() == [Some(0_u64); 4096].as_slice()`.

Author: bors